During operation of a gas turbine engine, foreign object debris (debris from outside the engine) and domestic object debris (debris from inside the engine) can contact (impact) a spinning rotor blade. The contact with the debris can cause instantaneous damage to the blade or impending blade failure. Instantaneous damage to the blade occurs when the debris causes a nick, scratch, bend or other observable permanent defect primarily due to the impact event itself. Impending failure occurs primarily during continuing engine operations. The impact event can also serve as an indicator of the primary or secondary debris travelling through the gaspath and potentially causing engine distress.
Detecting blade impact events which cause instantaneous blade damage is practical using standard blade-tip-timing (BTT) techniques, as the alteration in the performance of the rotor blades is permanent and repeats itself, but still the success of these techniques depends on damage size and location. In contrast, however, impacted rotor blades with impending failure can frequently return to their original geometry and performance relatively quickly. Existing sensor systems and signal processing systems frequently have difficulty identifying the impending damage, due to how quickly the blades return to their prior performance.
Prior systems can be broadly divided into two categories. In the first category, the output of the sensor arrangement is a sequence of blade pulses corresponding to observing (detecting) blade raw as rotor is spinning. Such systems lack the ability to output waveforms. Furthermore, an impacted blade can go undetected in such sensor arrangement, or can be represented by a pulse that is not easily recoverable from noise or distinguishable from a spurious pulse. For some rotors, direct detection of blade pulses limits the observation of all blades by a given sensor arrangement (this may occur if blade geometry varies between blades on the same rotor).
The second broad category includes sensor arrangements that output a full waveform, either directly, or after signal conditioning. The waveform can be processed to extract blade pulses and then the waveform is discarded.